New Scientist reports (“Dark matter no-show puts WIMPs in a bind“):
Despite tantalising early hints of a sighting, the most sensitive search yet for dark matter has come up empty. First results from the Large Underground Xenon (LUX) detector in South Dakota, announced today, failed to confirm previous potential sightings reported by other detectors. That may spell trouble for elegant recent theories of a shadow universe where myriad particles interact via their own dark forces.
Just to remind you: Dark Matter is the "invisible gravitational mass" that holds a galaxy together so that it spins like a plate and not a bowl of alphabet soup. From observations of stars moving around the center of the Andromeda galaxy, we know that 70 percent of the gravity in Andromeda is not from stars. But no matter what frequency we use — radio, microwave, infrared, x-ray, gamma-ray — we just don’t see the stuff we know is there. Hence the moniker "dark matter."
Two theories have been proposed:
- It is exotic matter like neutrinos that have mass but just don’t emit much light no matter how hot they are. Not only so, but they don’t push other matter around either, they can only be observed by their gravity. Hence the moniker WIMP for "Weakly Interacting Massive Particles."
- It is ordinary matter that is too cold to give off light — "cold dark matter." But it is sufficiently concentrated to not absorb light either, the way smoke or fog obscures the light of things behind it. And even though it is thermally cold, it has to be "kinetically" hot, or it would all fall into the center of the galaxy. Hence the acronym "MACHO" for "Massive And Compact [galactic] Halo Object." (Yeah, I know, but good acronyms are hard to make, especially that can pair with WIMP.)
So why is (1) preferred over (2)? Because (a) it gets money for particle detectors rather than astronomers — e.g., more support. (b) The Big Bang Nucleosynthesis (BBN) modellers say that their models can’t add any more normal ("baryonic") matter without having to rejigger their reductionist assumptions. (c) The astronomers who look for "microlensing" — where a moving black hole photobombs the picture of a distant star causing the distant star to momentarily brighten and wane — don’t see enough black holes for MACHO’s to explain the Andromeda data.
I prefer (2) over (1), simply because we have no evidence for "exotic matter." This paper, referenced by New Scientist, reports perhaps the fifth or sixth failed search: "First results from the LUX dark matter experiment at the Sanford Underground Research Facility." So why do they keep funding it? Same reason we fund evolutionary biology.
But there is an answer to (a),(b), and (c). (a) Build another accelerator and keep those particle physicists busy elsewhere. (b) Make the BBN modellers put reality back into their models — like adding magnetic fields and full 3D codes for example. (c) Make the MACHO’s smaller than black holes and bigger than dust. Dirty snowballs are just about the perfect size for MACHO’s to remain undetected.
Perhaps one day there will be funding for theories that don’t require a ten-institution consortium. One can only dream.
Image credit: Andromeda in Ultraviolet, NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP).